diff --git a/builtins.m4 b/builtins.m4
index 7aaa3546..957af297 100644
--- a/builtins.m4
+++ b/builtins.m4
@@ -1777,6 +1777,43 @@ prefetch_read(varying_int64, <$1 x i64>)
 prefetch_read(varying_float, <$1 x float>)
 prefetch_read(varying_double, <$1 x double>)
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; assert
+
+declare i32 @printf(i8*, ...)
+declare void @abort() noreturn
+
+define internal void @__do_assert_uniform(i8 *%str, i1 %test, <$1 x i32> %mask) {
+  br i1 %test, label %ok, label %fail
+
+fail:
+  %call = call i32 (i8*, ...)* @printf(i8* %str)
+  call void @abort() noreturn
+  unreachable
+
+ok:
+  ret void
+}
+
+
+define internal void @__do_assert_varying(i8 *%str, <$1 x i32> %test,
+                                          <$1 x i32> %mask) {
+  %nottest = xor <$1 x i32> %test,
+                 < forloop(i, 1, eval($1-1), `i32 -1, ') i32 -1 >
+  %nottest_and_mask = and <$1 x i32> %nottest, %mask
+  %mm = call i32 @__movmsk(<$1 x i32> %nottest_and_mask)
+  %all_ok = icmp eq i32 %mm, 0
+  br i1 %all_ok, label %ok, label %fail
+
+fail:
+  %call = call i32 (i8*, ...)* @printf(i8* %str)
+  call void @abort() noreturn
+  unreachable
+
+ok:
+  ret void
+}
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; stdlib transcendentals
 ;;
diff --git a/docs/ispc.txt b/docs/ispc.txt
index 53c2bfa2..f0659b48 100644
--- a/docs/ispc.txt
+++ b/docs/ispc.txt
@@ -89,6 +89,7 @@ Contents:
 
   + `Math Functions`_
   + `Output Functions`_
+  + `Assertions`_
   + `Cross-Program Instance Operations`_
   + `Converting Between Array-of-Structures and Structure-of-Arrays Layout`_
   + `Packed Load and Store Operations`_
@@ -1840,6 +1841,35 @@ values for the inactive program instances aren't printed.  (In other cases,
 they may have garbage values or be otherwise undefined.)
 
 
+Assertions
+----------
+
+The ``ispc`` standard library includes a mechanism for adding ``assert()``
+statements to ``ispc`` program code.  Like ``assert()`` in C, the
+``assert()`` function takes a single boolean expression as an argument.  If
+the expression evaluates to true at runtime, then a diagnostic error
+message printed and the ``abort()`` function is called.
+
+When called with a ``varying`` quantity, an assertion triggers if the
+expression evaluates to true for any any of the executing program instances
+at the point where it is called.  Thus, given code like:
+
+::
+
+    int x = programIndex - 2;  // (-2, -1, 0, ... )
+    if (x > 0)
+        assert(x > 0);
+
+The ``assert()`` statement will not trigger, since the condition isn't true
+for any of the executing program instances at that point.  (If this
+``assert()`` statement was outside of this ``if``, then it would of course
+trigger.)
+
+To disable all of the assertions in a file that is being compiled (e.g.,
+for an optimized release build), use the ``--opt=disable-assertions``
+command-line argument.
+
+
 Cross-Program Instance Operations
 ---------------------------------
 
diff --git a/ispc.cpp b/ispc.cpp
index ae6585c5..3a3d3751 100644
--- a/ispc.cpp
+++ b/ispc.cpp
@@ -283,6 +283,7 @@ Opt::Opt() {
     fastMath = false;
     fastMaskedVload = false;
     unrollLoops = true;
+    disableAsserts = false;
     disableBlendedMaskedStores = false;
     disableCoherentControlFlow = false;
     disableUniformControlFlow = false;
diff --git a/ispc.h b/ispc.h
index 2f952a54..ba5ce632 100644
--- a/ispc.h
+++ b/ispc.h
@@ -226,6 +226,10 @@ struct Opt {
         it will make sense. */
     bool unrollLoops;
 
+    /** Indicates whether assert() statements should be ignored (for
+        performance in the generated code). */
+    bool disableAsserts;
+
     /** On targets that don't have a masked store instruction but do have a
         blending instruction, by default, we simulate masked stores by
         loading the old value, blending, and storing the result.  This can
@@ -365,6 +369,7 @@ enum {
     COST_TYPECAST_SIMPLE = 1,
     COST_UNIFORM_LOOP = 4,
     COST_VARYING_LOOP = 6,
+    COST_ASSERT = 8,
 
     CHECK_MASK_AT_FUNCTION_START_COST = 16,
     PREDICATE_SAFE_IF_STATEMENT_COST = 6,
diff --git a/lex.ll b/lex.ll
index 3e622311..e752dc8d 100644
--- a/lex.ll
+++ b/lex.ll
@@ -78,6 +78,7 @@ ZO_SWIZZLE ([01]+[w-z]+)+|([01]+[rgba]+)+|([01]+[uv]+)+
 "/*"            { lCComment(yylloc); }
 "//"            { lCppComment(yylloc); }
 
+__assert { return TOKEN_ASSERT; }
 bool { return TOKEN_BOOL; }
 break { return TOKEN_BREAK; }
 case { return TOKEN_CASE; }
diff --git a/main.cpp b/main.cpp
index 5efb2118..72ec1355 100644
--- a/main.cpp
+++ b/main.cpp
@@ -83,6 +83,7 @@ static void usage(int ret) {
     printf("    [-o <name>/--outfile=<name>]\tOutput filename (may be \"-\" for standard output)\n");
     printf("    [-O0/-O1]\t\t\t\tSet optimization level (-O1 is default)\n");
     printf("    [--opt=<option>]\t\t\tSet optimization option\n");
+    printf("        disable-assertions\t\tRemove assertion statements from final code.\n");
     printf("        disable-loop-unroll\t\tDisable loop unrolling.\n");
     printf("        fast-masked-vload\t\tFaster masked vector loads on SSE (may go past end of array)\n");
     printf("        fast-math\t\t\tPerform non-IEEE-compliant optimizations of numeric expressions\n");
@@ -246,6 +247,8 @@ int main(int Argc, char *Argv[]) {
                 g->opt.fastMath = true;
             else if (!strcmp(opt, "fast-masked-vload"))
                 g->opt.fastMaskedVload = true;
+            else if (!strcmp(opt, "disable-assertions"))
+                g->opt.disableAsserts = true;
             else if (!strcmp(opt, "disable-loop-unroll"))
                 g->opt.unrollLoops = false;
 
diff --git a/module.cpp b/module.cpp
index 2881c810..1ddeaa55 100644
--- a/module.cpp
+++ b/module.cpp
@@ -928,6 +928,13 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
         FATAL("Unhandled target ISA in preprocessor symbol definition");
     }
 
+    if (g->includeStdlib) {
+        if (g->opt.disableAsserts) 
+            opts.addMacroDef("assert(x)=");
+        else
+            opts.addMacroDef("assert(x)=__assert(#x, x)");
+    }
+
     for (unsigned int i = 0; i < g->cppArgs.size(); ++i) {
         // Sanity check--should really begin with -D
         if (g->cppArgs[i].substr(0,2) == "-D") {
diff --git a/parse.yy b/parse.yy
index 1157ed17..12fb3fdf 100644
--- a/parse.yy
+++ b/parse.yy
@@ -161,8 +161,8 @@ static const char *lParamListTokens[] = {
 %token TOKEN_CASE TOKEN_DEFAULT TOKEN_IF TOKEN_ELSE TOKEN_SWITCH
 %token TOKEN_WHILE TOKEN_DO TOKEN_LAUNCH
 %token TOKEN_FOR TOKEN_GOTO TOKEN_CONTINUE TOKEN_BREAK TOKEN_RETURN
-%token TOKEN_CIF TOKEN_CDO TOKEN_CFOR TOKEN_CWHILE
-%token TOKEN_CBREAK TOKEN_CCONTINUE TOKEN_CRETURN TOKEN_SYNC TOKEN_PRINT
+%token TOKEN_CIF TOKEN_CDO TOKEN_CFOR TOKEN_CWHILE TOKEN_CBREAK
+%token TOKEN_CCONTINUE TOKEN_CRETURN TOKEN_SYNC TOKEN_PRINT TOKEN_ASSERT
 
 %type <expr> primary_expression postfix_expression
 %type <expr> unary_expression cast_expression launch_expression
@@ -177,7 +177,7 @@ static const char *lParamListTokens[] = {
 %type <stmt> statement labeled_statement compound_statement for_init_statement
 %type <stmt> expression_statement selection_statement iteration_statement
 %type <stmt> jump_statement statement_list declaration_statement print_statement
-%type <stmt> sync_statement
+%type <stmt> assert_statement sync_statement
 
 %type <declaration> declaration parameter_declaration
 %type <declarators> init_declarator_list 
@@ -1036,6 +1036,7 @@ statement
     | jump_statement
     | declaration_statement
     | print_statement
+    | assert_statement
     | sync_statement
     | error
     {
@@ -1193,6 +1194,13 @@ print_statement
       }
     ;
 
+assert_statement
+    : TOKEN_ASSERT '(' string_constant ',' expression ')'
+      {
+          $$ = new AssertStmt(*$3, $5, @1);
+      }
+    ;
+
 translation_unit
     : external_declaration
     | translation_unit external_declaration
diff --git a/stmt.cpp b/stmt.cpp
index bfd50950..b2c43b4d 100644
--- a/stmt.cpp
+++ b/stmt.cpp
@@ -755,6 +755,13 @@ lSafeToRunWithAllLanesOff(Stmt *stmt) {
     if ((ps = dynamic_cast<PrintStmt *>(stmt)) != NULL)
         return lSafeToRunWithAllLanesOff(ps->values);
 
+    AssertStmt *as;
+    if ((as = dynamic_cast<AssertStmt *>(stmt)) != NULL)
+        // While this is fine for varying tests, it's not going to be
+        // desirable to check an assert on a uniform variable if all of the
+        // lanes are off.
+        return false;
+
     FATAL("Unexpected stmt type in lSafeToRunWithAllLanesOff()");
     return false;
 }
@@ -1808,3 +1815,100 @@ PrintStmt::EstimateCost() const {
 }
 
 
+///////////////////////////////////////////////////////////////////////////
+// AssertStmt
+
+AssertStmt::AssertStmt(const std::string &msg, Expr *e, SourcePos p) 
+    : Stmt(p), message(msg), expr(e) {
+}
+
+
+void
+AssertStmt::EmitCode(FunctionEmitContext *ctx) const {
+    if (expr == NULL)
+        return;
+    const Type *type = expr->GetType();
+    if (type == NULL)
+        return;
+    bool isUniform = type->IsUniformType();
+
+    // The actual functionality to do the check and then handle falure is
+    // done via a builtin written in bitcode in builtins.m4.
+    llvm::Function *assertFunc = 
+        isUniform ? m->module->getFunction("__do_assert_uniform") :
+                    m->module->getFunction("__do_assert_varying");
+    assert(assertFunc != NULL);
+
+#ifdef ISPC_IS_WINDOWS
+    char errorString[2048];
+    if (sprintf_s(errorString, sizeof(errorString),
+                  "%s(%d): Assertion failed: %s\n", pos.name,
+                  pos.first_line, message.c_str()) == -1) {
+        Error(pos, "Fatal error in sprintf_s() call when generating assert "
+              "string.");
+        return;
+    }
+#else
+    char *errorString;
+    if (asprintf(&errorString, "%s:%d:%d: Assertion failed: %s\n", 
+                 pos.name, pos.first_line, pos.first_column, 
+                 message.c_str()) == -1) {
+        Error(pos, "Fatal error when generating assert string: asprintf() "
+              "unable to allocate memory!");
+        return;
+    }
+#endif
+
+    std::vector<llvm::Value *> args;
+    args.push_back(ctx->GetStringPtr(errorString));
+    args.push_back(expr->GetValue(ctx));
+    args.push_back(ctx->GetFullMask());
+    ctx->CallInst(assertFunc, args, "");
+
+#ifndef ISPC_IS_WINDOWS
+    free(errorString);
+#endif // !ISPC_IS_WINDOWS
+}
+
+
+void
+AssertStmt::Print(int indent) const {
+    printf("%*cAssert Stmt (%s)", indent, ' ', message.c_str());
+}
+
+
+Stmt *
+AssertStmt::Optimize() {
+    if (expr)
+        expr = expr->Optimize();
+    return this;
+}
+
+
+Stmt *
+AssertStmt::TypeCheck() {
+    if (expr)
+        expr = expr->TypeCheck();
+    if (expr) {
+        const Type *type = expr->GetType();
+        if (type) {
+            bool isUniform = type->IsUniformType();
+            if (!type->IsNumericType() && !type->IsBoolType()) {
+                Error(expr->pos, "Type \"%s\" can't be converted to boolean for \"assert\".",
+                      type->GetString().c_str());
+                return NULL;
+            }
+            expr = new TypeCastExpr(isUniform ? AtomicType::UniformBool : 
+                                                AtomicType::VaryingBool, 
+                                    expr, expr->pos);
+        }
+    }
+    return this;
+}
+
+
+int
+AssertStmt::EstimateCost() const {
+    return (expr ? expr->EstimateCost() : 0) + COST_ASSERT;
+}
+
diff --git a/stmt.h b/stmt.h
index c23c3071..1c7cc355 100644
--- a/stmt.h
+++ b/stmt.h
@@ -305,4 +305,29 @@ public:
 };
 
 
+/** @brief Representation of an assert statement in the program.
+
+    Like print() above, since we don't have strings as first-class types in
+    the language, we need to do some gymnastics to support it.  Like
+    assert() in C, assert checks the given condition and prints an error
+    and calls abort if the condition fails.  For varying conditions, the
+    assert triggers if it's true for any of the program instances.
+*/
+class AssertStmt : public Stmt {
+public:
+    AssertStmt(const std::string &msg, Expr *e, SourcePos p);
+
+    void EmitCode(FunctionEmitContext *ctx) const;
+    void Print(int indent) const;
+
+    Stmt *Optimize();
+    Stmt *TypeCheck();
+    int EstimateCost() const;
+
+    /** Message to print if the assertion fails. */
+    const std::string message;
+    /** The expression to be evaluated (that is asserted to be true). */
+    Expr *expr;
+};
+
 #endif // ISPC_STMT_H
diff --git a/tests/assert-1.ispc b/tests/assert-1.ispc
new file mode 100644
index 00000000..1bcf71c3
--- /dev/null
+++ b/tests/assert-1.ispc
@@ -0,0 +1,15 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    // Neither of these should hit!
+    assert(a > 0);
+    assert(b == 5.);
+    RET[programIndex] = 2*a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + 2*programIndex;
+}
diff --git a/tests/assert-2.ispc b/tests/assert-2.ispc
new file mode 100644
index 00000000..90fdea85
--- /dev/null
+++ b/tests/assert-2.ispc
@@ -0,0 +1,17 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    a -= 2;
+    if (a > 0)
+        // This shoudln't hit, given the if test above
+        assert(a > 0);
+    assert(b == 5.);
+    RET[programIndex] = a+2; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}
diff --git a/tests/assert-3.ispc b/tests/assert-3.ispc
new file mode 100644
index 00000000..06da5af4
--- /dev/null
+++ b/tests/assert-3.ispc
@@ -0,0 +1,17 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    a = -a;
+    if (a > 0)
+        // This shoudln't hit, given the if test above
+        assert(a > 0);
+    assert(b == 5.);
+    RET[programIndex] = -a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}
diff --git a/tests/assert-4.ispc b/tests/assert-4.ispc
new file mode 100644
index 00000000..a4e04dad
--- /dev/null
+++ b/tests/assert-4.ispc
@@ -0,0 +1,16 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    if (a < 0)
+        // Though this condition is false, the assert shouldn't hit, since
+        // given the if test above, all lanes should be off.
+        assert(b == 0.);
+    RET[programIndex] = a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}